This invention relates to cutting covers, locking mechanisms for such covers, and methods for making a cutter cover.
In certain types of known cutting systems, a workpiece is drawn between a first cylinder and a second cylinder. The first cylinder has cutting teeth mounted on it, so that when it rotates, the teeth cut into the workpiece. In order to ensure that the cutting extends all the way through the workpiece, the second cylinder, which rotates about an axis parallel to the axis of the cutting first cylinder, is mounted so that its circumference is near the blade. The second cylinder is covered by a protective cover (sometimes called a blanket) into which the teeth penetrate, so that the teeth clearly pass through the workpiece. The cover should be made of a material that can recover from the penetration of the teeth for at least some period of time. Covers and cutters of this general type are described in a number of patents, including U.S. Pat. Nos. 5,720,212; 5,758,560; 5,076,128; and 4,867,024.
Rotary die cutting covers are typically made by casting an elastomer, such as urethane, in an oversized mold that has a sheet of steel as a backing and aluminum end pieces. The casting is mounted on a rotating cylinder and ground down to size. Such a process wastes material and is labor intensive. The steel backing is designed to give stability to the urethane and to inhibit variations with thermal change. When the elastomer cools after the casting process, however, the elastomer shrinks. Variations in cooling can cause serious problems if the manufacture and use of the cover are performed under different conditions. While steel is desirable in some respects because it has a low thermal variation and is resilient, there are also drawbacks to its use. The urethane is cast onto the steel at a temperature of about 220xc2x0 F. (105xc2x0 C.). Thermal shrinkage for urethane due to the polymerization process is slight, but the shrinkage from 220xc2x0 F. (105xc2x0 C.) down to room temperature is significant, and causes the urethane to pull back. Because the urethane is bonded to the sheet of steel and the steel does not have large thermal variation, the two materials create stress that can be visible in the way the cover is shaped after it is cured. The cover may be cast in an arc, but it winds up flat after curing and cooling. At room temperature, it acts much like a bimetallic strip does, i.e., by bending towards the more thermally affected and therefore smaller material. The stresses on the cover also create a surface stress on the cover, causing cuts made in the material during operation to widen and tear more easily, and thus requiring more frequent replacement.
The cover is passed around the second cylinder, then forced to bend into a circle. The two ends of the cover are mounted in an axial slot in the cylinder where the ends are locked together. Since the urethane is stressed from a flat state into a circular state, force needs to be applied to keep the cover on the cylinder. This force becomes evident in the design of a lock in the slot for holding the cover on the cylinder. The lock requires so much material interference in order to assure that the cover will not open during use, that it may be difficult to use. Moreover, access to the second cylinder is often difficult, and the time involved to mount and remove the covers can thus be significant. The machines on which the covers are used are usually run continuously, and downtime is very undesirable in the cutting industry.
The ends of the locks can be bolted together. There are also a number of approaches for boltless locking, such as U.S. Pat. Nos. 5,720,212, 5,758,560, and 5,076,128. Each of these boltless locks requires a step or shoulder between male and female pieces such that one of the pieces is rotated under the other into place. Such locking mechanisms, after being locked in a slot, are difficult to unlock.
The present invention includes a cover, a process for forming a cover, a locking mechanism for a cover, methods for locking a cover, and cutting systems including such a cover and locking mechanism.
In one aspect, the invention includes a process of making a multi-layer cover with stresses that balance out and create a cover that, once post cured, is still curved into an arc. The curved cover makes easier mounting, removal, and use.
The process for making the cover includes centrifugally casting (also known as spin casting) a cover, preferably with three layers. An outer layer is a resilient and recoverable material such as urethane, a middle layer has higher durometer but lower cure temperature than the outer layer, such as a woven glass fiber with an encapsulating material to create a matrix with the glass fiber. A tougher, more resilient inner layer of material is cast over the middle layer to prevent cracking or breakage of the middle layer. This inner layer material has a high tensile strength with higher durometer than the outer material. The materials and the thicknesses of those materials are chosen so that the stresses balance each other, so that the cover is created with any desired arc or curve such that the arcuate shape is maintained. For example, the inner layer has greater shrinkage than a thicker outer layer. This arcuate shape is preferably nearly a full circle, e.g., about 340xc2x0 or more, and it maintains its shape at rest.
The spin casting process includes providing a material in a molten form into a centrifuge with a mold to form a desired shape. The material and mold are spun, e.g., at 500-750 RPM. Additional materials can be added after a time to a multi-layered structure.
In another aspect, the invention includes a multi-layered rotary die cover that has an outer material that can be cut into and yet provide some recovery from the penetration of the teeth; a middle layer more rigid than the outer layer and having a relatively low elongation percentage and a higher durometer than the outer layer; and inner layer with a high tensile strength and also with a higher durometer than that of the outer layer. The cover is formed to have little stress so that it stays in a curved or arcuate shape when made and at rest.
In another aspect, the invention includes a locking mechanism for connecting ends of a cover over a cylinder. The locking mechanism includes a male portion at a first end of a cover and a female portion at second end of the cover (or possibly an end of a separate cover). The female portion has one side connected to the second end and a mating portion extending away from the second end. This mating portion includes a pivotable tang that allows the male portion to be snap fit into place by being moved substantially only radially and without requiring that the male and female portions be rotated relative to one another. The male portion, when snap fit into place, is substantially immovable in a radial direction and is rigidly held by the tang to prevent wear.
The female portion preferably has an axial slot for receiving a deformable member that is between the female portion and the radial side of slot in the cylinder. The deformable member provides for improved rigidity in the slot in a circumferential direction, while not providing any force on a radial or axial direction.
For holding the cover to the male and female portions, the cover preferably has monolithic members in the cover extending toward the locking mechanism pieces. The members are preferably T-shaped and the locking member portions each have slots with tabs for receiving the T-shaped portion to substantially prevent movement in the circumferential or radial directions.
The present invention also includes methods for attaching and detaching the locking mechanism where a male portion of the locking mechanism can be snap fit with substantially only radial movement. The male portion is conveniently removed by prying back the tang so that the male portion can pop out.
By providing a cover with less stress in the die cutting section of the cover, there is better cut and tear resistance. The locking mechanism of the present invention allows for more convenient attachment and detachment of the cover from the cylinder, thus reducing the amount of down time when the cover needs to be replaced. Other features and advantages will become apparent from the following description of preferred embodiments, drawings, and claims.